1. Technical Field
The present invention relates to a current-supply control unit, a fusing device, an image forming apparatus, and a current-supply control method of controlling a heating element, and more particularly to a current-supply control unit for a heating unit to prevent overshooting and power shortage at the heating unit when a power supply is activated, a fusing device employing the current-supply control unit, an image forming apparatus employing the fusing device, and a current-supply control method of controlling the heating element.
2. Description of the Background Art
Electronic devices such as electro-photographic image forming apparatuses and other image forming apparatuses may have a heater such as a heating element used for fusing images on recording media. When such apparatuses detect an input of a power source, the heater is turned on. An amount of input power to the heater may be limited to a given value by setting a given duty cycle in view of the detected input voltage and a target temperature of the heater to prevent overshooting and power shortage at the heater. As for electro-photographic image forming apparatuses, fusing heaters can be controlled using a method that controls the duty cycle of voltage applied to the fusing heaters.
FIG. 1 shows a circuit configuration of a conventional fusing device 100 of an electro-photographic image forming apparatus. As shown in FIG. 1, the fusing device 100 includes, for example, a fusing heater 101, a relay 103, a fusing control circuit 104, an alternating current (AC) voltage detection circuit 105, and a control board 106. The control board 106 may include an application specific integrated circuit (ASIC) 107.
The fusing heater 101 is connected to a commercial alternating current (AC) power source 102 via the relay 103 to be supplied with heater-driving power for the fusing heater 101. Further, the fusing heater 101 is connected to the fusing control circuit 104serially. Further, the AC voltage detection circuit 105 is connected in parallel to the fusing heater 101. Specifically, the AC voltage detection circuit 105 is disposed between the relay 103 and the fusing control circuit 104, which is a stage before the fusing control circuit 104.
A signal detected by the AC voltage detection circuit 105 is input to the ASIC 107 of the control board 106. The ASIC 107 may correspond to a main controller. Based on the detection signal of the AC voltage detection circuit 105, the ASIC 107 selects a power-supply duty cycle to the fusing heater 101, and supplies a fusing control signal to the fusing control circuit 104 to control the fusing control circuit 104. FIG. 2 shows a timing chart for controlling a conventional fusing device, and FIG. 3 is a flow chart showing steps corresponding to the timing chart of FIG. 2.
As shown in FIGS. 2 and 3, when a main power source is set to ON (timing T1), a direct current (DC) power source is activated, a software processing is executed, and the relay 103 is set to ON (timing T2: step S201). When the relay 103 is set to ON, the voltage detection is started by activating the AC voltage detection circuit 105, and the AC/DC converting process is started using an AC/DC converter (step S202).
While the AC/DC converting process is conducted, the ASIC 107 sets the fusing heater 101 to ON state using a software start control after confirming the activation of the relay 103 (timing T3).
After completing the software start control, without setting a duty limit, the ASIC 107 outputs a fusing ON signal (ON signal of fusing heater) to set the fusing heater 101 at ON state (timing T4).
The ASIC 107 obtains DC converted by the AC/DC converter (step S203), and determines AC voltage based on a table stored in the ASIC 107 (step S204). Then, based on the AC voltage, the ASIC 107 sets or changes the upper limit of the duty cycle during which the fusing heater 101 is ON (step S205).
Based on such duty cycle, the ASIC 107 outputs the fusing ON signal or fusing heater ON signal (step S206), by which the fusing control circuit 104 is shifted to ON state while limiting the duty cycle (step S207), and the fusing heater 101 is set to ON state (timing T5).
Then, a temperature sensor such as a thermistor detects the heater temperature, and determines whether the heater temperature reaches a target temperature (step S208).
If the heater temperature does not reach the target temperature (S208: No), the process returns to step S206 and steps S206 to S208 are repeated. When the heater temperature reaches the target temperature, the fusing control circuit 104 is set to OFF state (step S209), and heater activation control is terminated.
JP-2006-039027-A discloses an image forming apparatus having a configuration to prevent overshooting and power shortage. In this configuration, even if the input voltage by a power source fluctuates, the input offset power when activating the heater can be maintained at a constant level, and the offset power can be changed depending on the target temperature of the heater, by which the temperature control may be conducted without overshooting and power shortage.
In the configuration described above, a voltage detector to detect the input voltage by the power source is disposed at a stage before the heater control circuit, in which the input voltage of the power source can be detected when the power source switch is set ON. However, the voltage detector cannot detect the actual voltage at the both ends of the heater, at which a voltage drop may occur when the heater is turned to ON. As such, the conventional heater activation control may be conducted using a detection voltage different from the actual voltage at the both ends of the heater, causing overshooting and power shortage at the heater.